Two piece center section for a drive apparatus

ABSTRACT

A center section or other mounting member for mounting and hydraulically connecting a pump and motor, includes a first piece having a running surface for the pump and connected to hydraulic porting and a second separable piece having a running surface for the motor and connected to the hydraulic porting. A spacer is disposed between the first piece and the second piece for hydraulically connecting the first piece and the second piece and for modifying the distance between the pump running surface and the motor running surface, whereby the first piece is physically separated from the second piece. Two such mounting members may be used at opposite ends of a main housing with a drive device having two pumps and two motors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 12/027,048 filed on Feb. 6, 2008, which is a continuation of U.S. patent application Ser. No. 11/117,912 filed on Apr. 29, 2005, now U.S. Pat. No. 7,392,654; which is a continuation-in-part of U.S. patent application Ser. No. 10/788,534 filed on Feb. 27, 2004, now U.S. Pat. No. 6,973,783. This application is also related to U.S. patent application Ser. No. 12/049,783 filed on Mar. 17, 2008, now U.S. Pat. No. 7,536,857; U.S. patent application Ser. No. 12/184,370 filed on Aug. 1, 2008, now U.S. Pat. No. 7,900,447 and U.S. patent application Ser. No. 13/041,016 filed on Mar. 4, 2011, now U.S. Pat. No. 8,327,639. All of these applications are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

This invention relates to hydraulic pump and motor assemblies.

SUMMARY OF THE INVENTION

A zero turn drive apparatus comprising dual tandem pumps and corresponding hydraulic motors is disclosed herein. The details of this invention are set forth below in connection with the detailed description of the embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vehicle employing a first embodiment of the drive assembly of the present invention.

FIG. 2 is a top view of a first embodiment of the drive assembly.

FIG. 3 is a cross-sectional front view of the drive assembly along the lines 3-3 in FIG. 2.

FIG. 4 is a cross-sectional view of the center section assembly along the lines 4-4 in FIG. 2.

FIG. 5 is an external view of a center section assembly in accordance with a first embodiment of this invention.

FIG. 6 is a cross-sectional front view similar to FIGS. 3 and 4 of a second embodiment of this invention.

FIG. 7 is a cross-sectional front view similar to FIGS. 3 and 4 of a third embodiment of this invention.

FIG. 8 is a partial cross-sectional front view of a fourth embodiment of this invention.

FIG. 9 is a detailed end view of an alternative embodiment of a center section assembly in accordance with the present invention.

FIG. 10 is a detailed side view of the embodiment of the center section assembly depicted in FIG. 9.

FIG. 11 is a detailed end view of a further alternative embodiment of the center section assembly in accordance with the present invention.

FIG. 12 depicts a vehicle employing a fifth embodiment of a drive assembly in accordance with the present invention.

FIG. 13 depicts a front view of the drive assembly shown in FIG. 12.

FIG. 14 depicts a front view of the drive assembly shown in FIG. 12 with the front cover of the housing removed.

FIG. 15 depicts a top view of the drive assembly shown in FIG. 12 with a portion of the drive assembly shown as a cross-sectional view.

FIG. 16 depicts a partial cross-sectional side view of the drive assembly shown in FIG. 13 taken along lines 6-16.

FIG. 17 depicts a front view of the drive assembly shown in FIGS. 12 and 15 taken along lines 17-17 in FIG. 15 with a portion of the drive assembly shown as a cross-sectional view.

DETAILED DESCRIPTION OF THE DRAWINGS

The following is at description of multiple embodiments of this invention. Where appropriate, like numerals indicate identical or substantially identical components, and similar numerals with a different initial numeral indicate similar components with certain differences as specified. Further, in each of the embodiments discussed herein, identical numerals followed by “a” and “b” identify elements that are either identical or are mirror images of each other. Therefore, for convenience, the descriptions of elements with numerals followed by “a” may apply equally to elements with identical numerals followed by “b.” Application of reference numerals in the aforementioned manner should have an obvious meaning to those with skill in the art.

FIGS. 1, 2 and 3 depict a pump and motor assembly 120 having a housing 52. FIG. 1 depicts an exemplary vehicle 115 having a frame 136 on which assembly 120 is mounted. Engine 116 is mounted to frame 136 and is coupled to assembly 120 by means of belt 119. Assembly 120 is likewise coupled to deck 117 by belt 219. Linkage 118 is connected to and actuates trunnion arm 21, which is described in detail below. It will be understood by one of skill in the art that FIG. 1 depicts an exemplary embodiment rather than a limiting one. Assembly 120 has many applications and is not limited to applications such as vehicle 115, nor is it limited to use with vehicles as there are industrial applications for such units

FIGS. 3, 6, 7 and 8 are cross-sectional views of different embodiments of this invention. It will be understood that, for clarity, certain elements of these figures are not shown in cross sectioning. By way of example, in FIG. 3 hydraulic motors 126 a and 126 b, center sections 122 a and 122 b, input shaft 54, gears 35 a, 35 b and 36, trunnions 21 a and 21 b and charge pumps 124 a and 124 b are not shown in cross section. The internal structure of such elements is generally known in the art and cross sectioning would not assist in the understanding of the invention. It will be understood, however, that different types of motors, pumps and center sections could be used in this invention, and that these items need not necessarily be identical on both sides of the unit.

In this embodiment, two generally identical pumps 11 a and 11 b are disclosed within unitary housing 52; as noted above, pinups 11 a and 11 b need not be identical and substantial variations are possible to one or the other within the scope of this invention. Pumps 11 a and 11 b are shown as the rotating axial piston type, although other designs could be used with minor modifications within the scope of this invention. Only certain elements of pump 11 a and its related structure are described, in detail herein, as the operation of such pumps is generally known in the art. The design of pumps 11 a and 11 b, housing 52 and related elements are similar to the disclosures in U.S. Pat. Nos. 6,705,840 and 7,056,101 both of which are commonly owned with this application, the terms of which are incorporated herein by reference.

Housing 52 includes two generally identical pump chambers or cavities 59 a and 59 b and a gear chamber or cavity 30 formed therebetween, thus providing an integral housing for pumps 11 a and 11 b. A case fluid passage 55 a is formed in housing 52 to connect pump chamber 59 a and gear chamber 30. Case fluid passage 55 b similarly connects pump chamber 59 b with gear chamber 30. Both fluid passages 55 a and 55 b permit hydraulic oil flow between pump chambers 59 a and 59 b and gear chamber 30. This arrangement permits the use of a single case drain (not show which can be formed anywhere in housing 52 to permit oil flow to a separate reservoir (not shown) mounted elsewhere on vehicle 115. If this case drain is in gear chamber 30, it will assist in preventing contamination of pumps 11 a and 11 b with debris from bevel gears 35 a, 35 b and 36, as well as assist in reduction of heat, as the warmer fluid from pumps 11 a and 11 b will be carried to the center of housing 52. A cover 53 is secured to the bottom of housing 52 to close gear chamber 30.

In each embodiment described herein, cavities 59 a, 59 h and 30 remain in fluid communication with one another. It will be appreciated by those in the art that pump chambers 59 a and 59 b and gear chamber 30 can be segregated from one another by housing 52. In such an embodiment, a plurality of case drains must be provided to remove oil from pump chambers 59 a and 59 b, respectively, and these separate case drains could be formed in a variety of locations.

Center sections 122 a and 122 b are mounted on opposite ends of housing 52 and act to seal pump chambers 59 a and 59 b, and may be secured thereto by screws 22 or other means. Center sections 122 a and 122 b could be referred to as porting plates, end caps or the like. As noted above, only center section 122 a will be described herein.

Preferably, motor 126 a is of the geroller type, but various other motor designs, such as axial piston motors and gear motors, may also be employed within the scope of this invention. Motors such as motor 126 a are also generally well known in the art and, therefore, it also is not described in detail. Motor 126 a is connected to center section 122 a by fasteners 130, which are typically screws, but other fastening, means may also be used. Motor 126 a is connected to and drives output axle 128 a, which in turn is connected to an output device, such as wheel 138.

Pump 11 a on the left hand side of FIG. 3 comprises a pump cylinder block 31 a having a plurality of axial pistons 38 a mounted therein and it is mounted on a running surface formed on center section 122 a.

Center section 122 a may be preferably composed of cast iron, although it could also be aluminum or other materials depending on the application needs. The use of running plate 37 a, also called a valve plate, to mount cylinder block 31 a will add additional strength and durability, particularly if center section 122 a is composed of aluminum. Cylinder block 31 a could also nm directly on a surface formed on center section 122 a. Pump 11 a is of the cradle mounted swash plate design; as shown in FIG. 3, swash plate 32 a is mounted in pump chamber 59 a on cradle bearings 46 a mounted on an inner wall of housing 52. Pistons 38 a run against swash bearing 33 a mounted in swash plate 32 a. Trunnion arm 21 a is engaged to a control block 45 a that is engaged to swash plate 32 a, so that rotation of trunnion arm 21 a causes movement of swash plate 32 a to various stroked forward or reverse positions, or to the neutral position.

While trunnion arms 21 a and 21 b are shown extending out of housing 52 on the same side as and thus parallel to input shaft 54, it will be understood that trunnion arms 21 a and 21 b could be mounted on any side of housing 52, possibly requiring a corresponding change in the orientation of swash plates 32 a and 32 b, respectively, and also possibly requiring a change in the orientation of the porting in center sections 122 a and 122 b, respectively. Trunnion arms 21 a and 21 b need not be on the same side of housing 52. By way of example, if trunnion arm 21 a is rotated 180° from the orientation shown, housing 52 would need to be modified, but the orientation of porting in center section 122 a would not need to be changed. If, however, trunnion arm 21 a was rotated 90° from the orientation shown so that it was on an adjacent side of housing 52, the porting in center section 122 a would also need to be similarly rotated, along with the proper housing modifications. It will also be understood that other types of swash plates 32 a and 32 b, such as a trunnion mounted swash plate, could be used.

As will be obvious to one of ordinary skill in the art, optional elements, such as an oil cooler, external reservoir or expansion tank could easily be attached to pump and motor assembly 120. Various fittings and connections, such as housing or center section ease drains and appropriate hydraulic lines, would be used to connect these elements to assembly 120 and to each other. These elements have been removed to simplify the drawings. Input shaft 54 extends into housing 52; as shown in FIG. 1, it can be driven by a prime mover such as engine 116 through pulleys 51 and belt 119, or some other means. In the embodiment shown in FIG. 3, input shaft 54 extends through both housing 52 and cover 53, which includes bearing 57 therein to support input shaft 54. An output pulley 58 may be attached to the end of shaft 54 to drive an auxiliary device, such as mower deck 117 as shown in FIG. 1 or other device. Cover 53 is strengthened to support bearing 57 used to rotatably support shaft 54 and the torque loads from output pulley 58.

Bevel gear 36 is mounted on input shaft 54 inside gear chamber 30 and is drivingly engaged to a first driven bevel gear 35 a mounted on first pump shaft 63 a. Bevel gear 36 is likewise drivingly engaged to a second driven bevel gear 35 b which is similarly mounted on and driving second pump shaft 631. A benefit of this design is that pump shafts 63 a and 63 b can be sized appropriately for their respective pumps 11 a or 11 b; only input shaft 54 needs to be sized appropriately to handle the torque of both pumps 11 a and 11 b as well as the torque requirements of the auxiliary device attached to output pulley 58.

Pump shaft 63 a extends from gear chamber 30 into first pump chamber 59 a and is engaged to and drivingly rotates pump cylinder block 31 a. Bearing 44 a provides support within housing 52. Pump shaft 63 b also extends from gear chamber 30 into pump chamber 59 b where it engages and drivingly rotates pump cylinder block 31 b in a similar manner. As shown in, e.g., FIG. 3, input shaft 54 is generally perpendicular to pump shafts 63 a and 63 b and extends out the side of housing 52 as opposed to the ends thereof, which provides the user with flexibility in the application.

Pump shaft 63 a extends through center section 122 a into charge pump 124 a. Charge pump 124 a can be a gerotor or other style of charge pump, such as a vane pump, geroller, gear pump or any other known design. A charge pump for use in connection with a hydrostatic pump is shown, e.g., in commonly owned U.S. Pat. Nos. 5,555,727 and 5,628,189 the terms of which are incorporated herein by reference.

The location of input shaft 54 on the side of housing 52 permits the location of charge pumps 124 a and 124 b on opposite ends of housing 52. It is possible that only one of charge pumps 124 a or 124 b would be required, depending on the application for which apparatus 120 is to be used. Similarly, the output of one charge pump mounted on one center section could be attached by means of internal or external hoses or integral passages to provide charge pressure the other pump associated with the other center section.

It will also be understood that these embodiments could include additional gear reduction. For example, in FIG. 3, a gear reduction could be used between bevel gear 36 on input shaft 54 and bevel gear 35 a on pump shaft 63 a. Likewise, a gear reduction could be used between bevel gears 36 and 35 b. Furthermore, it will be understood that bevel gears 35, 36 a and 36 b could be replaced with another means for creating a right angle turn of the rotational force, such as helical gears, a worm gear driving, a spur gear and the like.

Details of center section 122 a are shown in FIGS. 4 and 5. Fluid for charge pump 124 a is introduced into center section 122 a through charge inlet 47 a, which may further be connected to a separate reservoir (not shown) or may be connected to pump chamber 59 a. Pressurized fluid from charge pump 124 a is introduced through one or more passages 60 a formed in center section 122 a into charge gallery 49 a. Pump 11 a is connected to motor 126 a through porting 41 a and 42 a, which may be closed by plugs 39 a. As pump 11 a is operated, one of porting 41 a or 42 a will be under pressure, and the other of porting 41 a or 42 a will be at vacuum pressure or at charge pressure. Hydraulic fluid leaks from a variety of places during operation of pump and motor assembly 120. The lost fluid is replaced when a check valve 67 a or 69 a opens in response to vacuum pressure or to pressure from charge pump 124 a, permitting replacement or makeup fluid to enter porting 41 a or 42 a, respectively.

Because pump 11 a, porting 41 a, porting 42 a, and motor 126 a form a closed loop, and because there may be a need to back drive motor 126 a, such as may occur if engine 116 becomes non-functional, a bypass valve 66 a may be provided. Bypass valve 66 a is typically positioned so that fluid flow in ports 41 a and 42 a is relatively unimpeded, for example, through annular areas 61 a and 62 a. When bypass valve 66 a is actuated, fluid is permitted to flow between ports 44 a and 42 a by way of passages 64 a formed in valve 66 a, so that motor 126 a is able to turn more freely.

FIG. 5 depicts an exterior view of center section 122 a. Charge pump 124 a is mounted to surface 50 a. Motor 126 a is mounted to surface 56 a.

FIG. 6 shows another embodiment of this invention, which is structurally and substantially similar to the embodiment shown in FIG. 3. FIG. 6 shows a pump and motor assembly 140, where the key differences from FIG. 3 center around input shaft 142. Specifically, in FIG. 6, input shaft 142 is driven by its prime mover 116 through pulley 144, which is located on the bottom of the unit instead of the top. Cooling fan 146 is attached to input shaft 142 at the end opposite from pulley 144. The use of such a fan can, in certain applications, eliminate the need for an oil cooler. This ability to modify the drive input location again improves the overall flexibility of the assembly.

FIG. 7 shows yet another embodiment of the invention which is also substantially similar to the embodiments depicted in FIGS. 3 and 6. Pump and motor assembly 160 includes an input shaft 168 coupled through coupler 166 to engine 162 output shaft 164. Coupler 166 may be of known design, such as using a powdered metal part with splines to interlock the two shafts 168 and 164; it could also be a cut steel part with a broached inner diameter to form the interlock. Such an embodiment eliminates the need for the pulleys shown in other embodiments and again increases the overall flexibility of the assembly constructed in accordance with the present invention. It will be appreciated by those in the an that either design, or any such similar design, will achieve the intended function.

Input shaft 168 does not extend through cover 153 in this embodiment and therefore cover 153 need not be additionally strengthened to support the torque loads from an output pulley, although this design could be modified to have such a pass through design as is shown in FIG. 3. In addition, center section 122 b as shown in FIG. 7 has a different length than center section 148 b as shown in FIG. 6. This difference in length varies the distance between output shafts 128 a and 128 b, as shown in FIG. 6, and pump shafts 63 a and 63 b. The length between output shafts 128 a and 128 b and pump shafts 63 a and 63 b can be varied depending on customer demand. The differing lengths are depicted as D1 in FIG. 6 and D2 in FIG. 7.

FIG. 8 shows a further embodiment of the invention that is structurally and substantially similar to the embodiment depicted in FIG. 6 FIG. 8 depicts a pump and motor assembly 180, wherein the key difference between the embodiments depicted in FIG. 6 and FIG. 8 is that motors 184 a and 184 b, which are attached to center sections 182 a and 182 b, are of the combination motor and gear reduction type that are generally described in commonly assigned U.S. Pat. No. 6,811,510, which is herein incorporated by reference. For convenience, only certain portions of motors 184 a and 184 h are described herein. Motor shaft 200 a extends through center section 182 a as depicted in FIG. 8. It is driven by cylinder block 201 a, and is coupled to ring/planet gear assembly 202 a. Gear assembly 202 a is drivingly engaged to axle shaft 186 a. An advantage of this embodiment is that a small efficient hydraulic motor and gear reduction may be used in place of a relatively inefficient all hydraulic reduction, as is typically used. Axle 186 a is connected to an output, such as wheel 138 shown in FIG. 1.

In each of the previous embodiments, the center sections have each been depicted as being a one piece unit. FIGS. 9 and 10 depict an end view and side view, respectively, of an alternative embodiment of center section assembly 230, where pump portion 232 is separate and removable from motor portion 234. Portions 232 and 234 are affixed by fasteners 240, which may be screws, bolts, or any other similar fasteners. Dowels 242 may also be inserted into both portions 232 and 234 to further stabilize center section assembly 230. Assembly 230 is affixed to housing 52 by fasteners 22, which are inserted into openings 236. Motor 126 a is affixed to motor portion 234 of assembly 230 by fasteners 130, which are inserted into openings 238. To prevent leakage, o-rings 244 are placed at porting where portions 232 and 234 join.

One advantage of this embodiment is that the distance, as shown as D1 in FIG. 6 and D2 in FIG. 7, between the pump shafts 63 and output shafts 128 or 186 can easily be varied for a variety of vehicle applications that might otherwise require multiple center sections of varying lengths. As shown in FIG. 11, center section assembly 250, which can be otherwise substantially similar to assembly 230 as shown in FIGS. 9 and 10, has spacer 254 is interposed between portions 232 and 234. Fasteners 252 are inserted through openings 256 and engage motor portion 234 to join pump portion 232, spacer 254 and motor portion 234 to one another. Dowels 242 may be inserted into pump portion 232, spacer 254 and motor portion 234 to further stabilize assembly 250. In order to prevent oil leakage, O-rings 244 may be placed at porting where pump portion 232 and spacer 254 join and where spacer 254 and motor portion 234 join.

Another embodiment of the present invention is shown in FIGS. 12-17. Similar to the first embodiment of the present invention described above and shown in FIGS. 1-5, this embodiment of pump and motor assembly 320 includes an engine 316, also known as a prime mover, mounted to frame 336, where frame 336 also supports assembly 320, mower deck 317 and other possible attachments. For attaching assembly 320 to engine 316, a bell housing 310 may be positioned between and secured to engine 316 and assembly 320. Similar to the embodiment shown in FIG. 7, main input shaft 354 of assembly 320 is coupled to the output shaft of engine 316. This embodiment also includes pumps 11 a and 11 b and center sections 122 a and 122 b, which are preferably nearly identical to those described above. In addition, trunnion anus 21 a and 21 b, swash plates 32 a and 32 b, and gears 35 a, 35 b and 36 are each preferably substantially similar to the similarly numbered elements described above and operate in the same manner.

For powering a mower deck 317, deck lift or other auxiliary device, a power take off assembly 170 may also be provided. As shown in FIGS. 12, 15 and 16, power take off assembly 170 may be positioned on the opposite side of assembly 320, as compared to bell housing 310. For driving power take off assembly 170, main input shaft 354 may extend outside of housing 152. Power take off assembly 170 also includes an output shaft 175, which may be drivingly coupled to right angle gear box 318 for powering mower deck 317. To actuate power take off assembly 170, an electric clutch 171, having a power connection 177 to connect it to the vehicle battery or other source of power, may be associated with main input shaft 354 and act to couple first pulley 172 to main input shaft 354. Furthermore, first pulley 172 may be drivingly coupled by belt 174 to a second pulley 173, which is fixedly attached to and drives output shaft 175. Output shaft 175 may be positioned below main input shaft 354 and may form a substantially collinear relationship with gear box 318, which will provide for a more efficient transfer of power from output shaft 175 to gear box 318 and mower deck 317. For supporting one end of main input shaft 354 and to protect electric clutch 171, pulleys 172, 173, output shaft 175 and main input shaft 354 from water and other debris, power take off cover 176 may be provided. Power take off cover 176 may be secured to housing 152 so as to create an environmental seal therewith or to create a water tight seal, as needed. It should be appreciated by those with skill in the art that electric clutches are common in the industry and that other variations of clutches may also be used to couple first pulley 172 to main input shaft 354.

While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangement disclosed is meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any equivalents thereof. 

What is claimed is:
 1. A drive device including a rotating pump cylinder block and a rotating motor cylinder block, the drive device comprising: a main housing having at least one of the cylinder blocks disposed therein; and a center section engaged to the main housing and having hydraulic porting formed therein for connecting the pump cylinder block and the motor cylinder block, the center section comprising: a first piece having a pump running surface connected to the hydraulic porting, wherein the pump cylinder block is rotatably disposed on the pump running surface; a second piece having a motor running surface connected to the hydraulic porting, wherein the motor cylinder block is rotatably disposed on the motor running surface; and a spacer disposed between the first piece and the second piece for hydraulically connecting the first piece and the second piece and for modifying the distance between the pump running surface and the motor running surface, whereby the first piece is physically separated from the second piece by the spacer.
 2. The drive device of claim 1, wherein the pump cylinder block is disposed inside the main housing and the motor cylinder block is disposed external to the main housing.
 3. The drive device of claim 2, wherein the main housing forms a sump for the pump cylinder block, and the first piece is secured to an external surface of the main housing to form a closure for the sump.
 4. The drive device of claim 1, further comprising a first plurality of dowels between the first piece and the spacer and a second plurality of dowels between the second piece and the spacer.
 5. The drive device of claim 1, wherein the first piece comprises a first generally planar side on which the pump running surface is located and a second generally planar side opposite to the first generally planar side.
 6. The drive device of claim 5, further comprising a charge pump disposed on the second generally planar side.
 7. The drive device of claim 5, wherein the second piece comprises a third generally planar side on which the motor running surface is located and a fourth generally planar side apposite to the third generally planar side.
 8. The drive device of claim 7, wherein the first generally planar side is generally coplanar with the fourth generally planar side, so that the pump cylinder block and the motor cylinder block are on opposite sides of the center section.
 9. The drive device of claim 7, further comprising a pump shaft engaged to and driving the pump cylinder block and having a first axis of rotation, and a motor shaft engaged to and driven by the motor cylinder block and having a second axis of rotation that is generally parallel to the first axis of rotation.
 10. The drive device of claim 9, further comprising a motor housing connected to the fourth generally planar side, wherein the motor cylinder block is disposed in the motor housing and is located external to the main housing.
 11. The drive device of claim 1, further comprising a pump shaft engaged to and driving the pump cylinder block and having a first axis of rotation, and a motor shalt engaged to and driven by the motor cylinder block and having a second axis of rotation that is generally parallel to the first axis of rotation.
 12. A drive device including a main housing, the drive device comprising: a first hydraulic pump hydraulically connected to a first hydraulic motor, and a second hydraulic pump hydraulically connected to a second hydraulic motor; a first center section engaged to the main housing and having a first pump miming surface for the first hydraulic pump, and a second center section engaged to the main housing and having a second pump running surface for the second hydraulic pump; wherein the first center section comprises a first pump section on which the first pump running surface is located, a first motor section having a first motor running surface for the first hydraulic motor and a first spacer disposed between the first pump section and the first motor section for modifying the distance between the first pump running surface and the first motor running surface; and wherein the second center section comprises a second pump section on which the second pump running, surface is located, a second motor section having a second motor running surface for the second hydraulic motor and a second spacer disposed between the second pump section and the second motor section for modifying the distance between the second pump running surface and the second motor running surface.
 13. The drive device of claim 12, wherein the main housing forms a sump for the first hydraulic pump and the second hydraulic pump, and the first pump section is secured to a first external surface of the main housing and the second pump section is secured to a second external surface of the main housing, thereby forming separate closures for the sump.
 14. The drive device of claim 13, wherein the first external surface is located at a first end of the main housing, and the second external surface is located at a second end of the main housing, opposite the first end.
 15. The drive device of claim 14, wherein the first hydraulic motor and the second hydraulic motor are both disposed external to the sump.
 16. The drive device of claim 12, further comprising: a first pump shaft engaged to and driving the first hydraulic pump and a second pump shaft engaged to and driving the second hydraulic pump, the first and second pump shafts having a common first axis of rotation; and a first motor shaft engaged to and driven by the first hydraulic motor and a second motor shaft engaged to and driven by the second hydraulic motor, the first and second motor shafts having a common second axis of rotation that is generally parallel to the first axis of rotation.
 17. A center section having hydraulic porting formed therein, for mounting and hydraulically connecting a pump cylinder block and a motor cylinder block, the center section comprising: a first piece having a pump running surface connected to the hydraulic porting, wherein the pump cylinder block may be rotatably disposed on the pump running surface; a second piece having a motor running surface connected to the hydraulic porting, wherein the motor cylinder block may be rotatably disposed on the motor running surface; and a spacer disposed between the first piece and the second piece for hydraulically connecting the first piece and the second piece and for modifying the distance between the pump running surface and the motor running surface, whereby the first piece is physically separated from the second piece by the spacer.
 18. The center section of claim 17, wherein the first piece comprises a first generally planar side on which the pump running surface is formed and a second generally planar side opposite to the first generally planar side.
 19. The center section of claim 18, wherein the first piece has further comprises a mounting surface for a charge pump on the second generally planar side.
 20. The center section of claim 18, wherein the second piece comprises a third generally planar side on which the motor running; surface is formed and a fourth generally planar side opposite to the third generally planar side, wherein the first generally planar side is generally coplanar with the fourth generally planar side, so that the pump running surface and the motor running surface are on opposite sides of the center section. 